Harry keeps following the Half-Blood Prince's instructions with much success. Even though he looks through the rest of the book he can't find out the identity of the Half-Blood Prince.

On his way to his first private meeting with Dumbledore, he hides when Professor Trelawney passes by reading cards. Dumbledore tells Harry in his private lessons he will be given certain information about Voldemort's life and history and why he attempted to kill Harry. He says many parts will be speculation, and it's even possible in some things he might be as wrong as Humphrey Belcher who thought the time was ripe for a cheese cauldron.

They use the Pensieve to go into a memory of Bob Ogden who was employed by the Department of Magical Law Enforcement. Ogden is going to some shabby house between Great Hangleton and Little Hangleton. He is told he is not welcome by someone speaking in Parseltongue. That person is Morfin Gaunt and his father Mr Gaunt reluctantly invites Ogden into the house. Ogden is looking for Morfin because of a breach of wizarding law, an attack on a muggle. In the house is also Morfin's younger sister Merope who is shy and being bullied by her father. It looks like she is a Squib, she doesn't manage to magically mend a pot she breaks and her father repairs it with a ¤ Reparo. Morfin needs to appear at a hearing. Mr Gaunt flaunts his heritage, showing a black-stoned golden ring with the Peverell coat of arms engraved on the stone. He also sticks a gold locket, which Merope is wearing, in his face. It is Salazar Slytherin's, they are his last living descendants. Ogden states this is beside the point and that Morfin still needs to appear on the hearing. At that moment a girl called Cecilia and a boy called Tom. Morfin tells Mr Gaunt that Merope likes the muggle, Tom, and Mr Gaunt gets furious. He starts choking Merope and Ogden uses a ¤ Relashio to stop him. Ogden needs to run for his life.

Harry and Dumbledore return from Ogden's memory. Dumbledore explains that Tom is Voldemorts father Tom Riddle (Senior) and Mr Gaunt, Morfin and Merope's father, is Marvolo Gaunt, Voldemorts grandfather. Merope is his mother. Morfin and Marvolo were overpowered fifteen minutes later when Ogden came back with reinforcements, and both convicted by the Wizengamot and sent to Azkaban, Morfin for three years and Marvolo six months.

Dumbledore thinks Merope used a love potion on Tom Riddle. Marvolo died soon after being released from Azkaban and finding his daughter having fled from home. Tom Riddle after a while, maybe because Merope thought Tom would not need a potion anymore to keep loving her, left Merope and claimed he was "hoodwinked". He never saw her again and never troubled to discover what became of the child to come, Merope was already pregnant at that point.

Harry may tell Ron and Hermione but they should keep everything to themselves. Dumbledore confirms the ring he was wearing is the same which Marvolo was showing in the memory, he acquired it very recently, 'around the time that he injured his hand'.